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4-20mA to PIC ADC

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Suraj143

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I need to interface six 4-20mA sensors with PIC ADC & scale to 1-5V range.

I need to protect the PIC input against the robost world,What will be the best circuit?Any modifications?
 
I need help guys.

hi,
The upper of the two circuits would be my option, take care the knee of the 5.1V zener does not clip at less that 5V.

The 10K together with the PIC's internal clamping diodes will protect the input pins
 
Hi eric I'll remove that zenner diode.

Here is the new schematic.I also added a 0.1uF cap near AD input.

Also what about adding a 100mA fuse in the current loop.

Is my diagram ok?
 
Hi eric I'll remove that zenner diode.

Here is the new schematic.I also added a 0.1uF cap near AD input.

Also what about adding a 100mA fuse in the current loop.

Is my diagram ok?

hi,

A 100mA fuse would not protect the 20mA current source, what environment is the loop working in?
and over what distance?
also are you using screened cable on your loop cable.

E
 
100mA fuse is used because,some faulty 4-20mA sensors will out above 1A.So it will damage the 250R.

Distance is 5M from sensor to controller.

Cable I don't know what cable is.
 
to get 1A through 250 Ohm you need 250V. all 4-20mA loops are powered by much lower voltage such as 24V. at 24V even if sensor is short circuited, current will not exceed some 100mA.
 
to get 1A through 250 Ohm you need 250V. all 4-20mA loops are powered by much lower voltage such as 24V. at 24V even if sensor is short circuited, current will not exceed some 100mA.

You got it now I understood.

My supply is 24V.

Even if the sensor shorted
V=IR
24=I X 250
96mA=I

So no point of adding a 100mA fuse.So what value do you recommend?Or do I need a fuse?
 
If your concern is damage to the 250 ohm resistor due to a shorted sensor, I would suggest just using a higher power resistor.

The 10K part will protect the PIC even from 24 volts across Rsense.

As for the capacitor at the input to the PIC, make sure you take into consideration the delay that it will add to the signal.
 
If your concern is damage to the 250 ohm resistor due to a shorted sensor, I would suggest just using a higher power resistor.

The 10K part will protect the PIC even from 24 volts across Rsense.

As for the capacitor at the input to the PIC, make sure you take into consideration the delay that it will add to the signal.

Thanks ChrisP..Now I cleared all my doubts.I'll use the last schematic.

Thanks fellows :)
 
sometimes people add fuse simply for convenience to test loop, not to protect it. when you plug multimeter across fuse and then open fuse, you can verify if reading of your device is correct without breaking the circuit. an example of situation where you would rather not break circuit is if data is logged or recorded.
 
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